When it comes to unmanned aerial vehicles (UAV), hydrogen is all the rage.

The first hydrogen-powered, unmanned UAV flight took place over California yesterday, AeroVironment, a company with 20 years experience in building unmanned aircraft, announced. Dubbed Global Observer, the "unmanned aircraft system" took off from Edwards Air Force Base and lasted more than four hours in the air. The company said the aircraft was able to reach an altitude of 5,000 feet.

Last year, Global Observer was put to the test at NASA's Dryden Flight Research Center. The government organization load tested Global Observer's wings, as well as ground vibration. NASA said at the time that the aircraft will eventually be used for "defense and homeland security missions." The organization could also see it used in "civil and commercial applications," like hurricane tracking and aerial imaging.

But before that happens, AeroVironment has much more testing to perform. The company said it "will now systematically expand the altitude and duration of test flights to validate the aircraft's high-altitude, long endurance performance." Ultimately, Global Observer should be able to fly at 55,000 to 65,000 feet for up to one week at a time. At that level, AeroVironment said, it should "view a significantly larger area on the surface of the earth than conventional, lower-flying aircraft."

The unmanned aircraft is designed to be more "reliable and more affordable" than UAVs already being used by the U.S. government. And thanks to its use of hydrogen, AeroVironment is quick to point out that Global Observer does not consume fossil fuels or emit carbon emissions.

However, AeroVironment isn't alone in developing hydrogen-powered UAVs. Last year, Boeing offered up some information on its Phantom Eye aircraft. Boeing's drone is capable of flying at 65,000 feet and cruise for up to four days.

If Global Observer and Phantom Eye testing goes as planned, it might not be long before the U.S. government's surveillance drone program enters its next phase. The Predator and Global Hawk, which are currently employed by the United States and have proven extremely successful, lack the improved surveillance capabilities and endurance of their hydrogen-powered counterparts.